Photoinitiated cationic polymerization has received considerable attention in recent years as way to prepare 100% reactive coatings, inks, and adhesives. Accordingly, various cationic initiators have been developed, many of which are based on sulfonium and iodonium organic salts. However, most iodonium organic salts have been found to be unstable in the presence of highly reactive monomers. Of the sulfonium salts, those found to be useful include the triaryl sulfonium salts described in U.S. Pat. No. 4,374,066, dialkyl phenacyl sulfonium salts described in U.S. Pat. Nos. 4,058,401 and dialkyl hydroxyaryl sulfonium salts described in U.S. Pat. No. 4,230,814. Although useful for initiating the polymerization of a number of cationically polymerizable resins including epoxides, cyclic ethers and vinyl ethers, these highly polar, crystalline initiators are not readily soluble in non-polar solvents. Therefore, they are unsuitable for initiating the polymerization of long chain resins such as long chain alpha-olefin oxides and alkyl vinyl ethers.
Recently, the preparation and use of triphenyl alkoxy sulfonium salt photoinitiators has been described in U.S. Pat. No. 4,882,201. Since it is now determined that hydrocarbon solubility improves with increasing length of an alkoxy chain group, these salts may be more suitable for initiating the cationic polymerizations of non-polar monomers and oligomers; however, increasing the alkyl chain length also increases crystallinity as indicated by a correspondingly increasing melting point. Hence, it is found that patentees' photoinitiators with long alkoxy chains must be heated to relatively high temperatures in order to effect their dissolution; thus, they are unsuitable for formulations involving volatile resins. A further drawback is that, since these initiators are only soluble in hydrocarbon resins at elevated temperatures, they tend to recrystallize out of solution at room temperature. The multicyclic alkaryl sulfonium salts of U.S. Pat. No. 4,319,974 also have been disclosed as cationic initiators; however, these display the same inability to remain in solution and recrystallize at or about room temperature.
Accordingly, it is an object of this invention to provide a cationic initiator having markedly improved solubility in long chain hydrocarbons or hydrocarbon ethers at room temperature.
Another object is to provide a cationic initiator which does not crystallize from a coating formulation at room temperature and which is effectively employed in a polymerization system involving hydrocarbon or volatile resins.
Another object is to provide a process for curing alpha-olefin and glycidyl ether monomers or oligomers under mild conditions which is both economical and convenient.
These and other objects of the invention will become apparent from the following description and disclosure.